FIRST LASING OF THE CAEP THZ FEL FACILITY DRIVEN BY A

SUPERCONDUCTING ACCELERATOR∗

D. Wu1, M. Li1,†, X. F. Yang1, X. J. Shu2, X. Y. Lu3, H. W. Huang4, Y. H. Dou2 H. B. Wang1,

X. Luo1, X. M. Shen1, D. X. Xiao1, J. X. Wang1, P Li1, X. K. Li1, K. Zhou1, C. L. Lao1,

Y. Xu1, P. Zhang1, L.G. Yan1, S. F. Lin1, Q. Pan1, L. J. Shan1, T. H. He1, W. Bai1, L. D. Yang1,

D. R. Deng1, H. Zhang1, J Liu1, Y. N. Chen1, D. C. Feng11Institute of Applied Electronics, China Academy of Engineering Physics, Mianyang, 621900, China

2 Institute of Applied Physics and Computational Mathematics, Beijing 100094, China3 Institute of Heavy Ion Physics, Peking University, Beijing 100871, China

4 Department of Engineering Physics, Tsinghua University, Beijing 10084, China

Abstract

China Academy of Engineering Physics terahertz free

electron laser (CAEP THz FEL, CTFEL) is the first THz

FEL oscillator in China, which was jointly built by CAEP,

Peking university and Tsinghua university. The stimulated

saturation of the CTFEL was reached in August, 2017. CT-

FEL consists of a GaAs photocathode high-voltage DC gun,

a superconducting RF linac, a planar undulator and a quasi-

concentric optical resonator. The terahertz laser’s frequency

is continuously adjustable from 2 THz to 3 THz. The aver-

age power is about 20 W and the micro-pulse power is more

than 0.3 MW.

INTRODUCTION

Free electron lasers (FELs) can be the most powerful

tool as terahertz power sources. It has many advantages,

such as monochrome, high-power, linear-polarization and

continuously-tunable frequency. Many FEL facilities, such

as ELBE in Germany [1], FELIX in Holland [2], UCSB

in the USA [3] and NovoFEL in Russia [4], have played

important roles in the THz sciences. In the near future, more

than 20 FEL facilities have been planned to build in the

whole world, of which there will be at least 8 ones operating

in the THz range [5].

CAEP THz FEL (CTFEL) facility is the first high average

THz source based on FEL in China [6, 7], which is driven

by a DC gun with a GaAs photocathode [8, 9] and two 4-cell

1.3 GHz super-conducting radio frequency (SRF) accelerator

[10, 11]. The repetition rate of CTFEL is 54.167 MHz, one

over twenty-four of 1.3 GHz. The effective accelerating field

gradient is about 10 MV/m.

CTFEL has achieved the stimulated saturation in August,

2017[12, 13]. The terahertz wave frequency is continuously

adjustable from 2 THz to 3 THz. The average power is about

21 W and the micro-pulse power is more than 0.3 MW. This

paper gives an introduction of this facility and its THz laser

characters.

∗ Work supported by China National Key Scientific Instrument and Equip-

ment Development Project (2011YQ130018), National Natural Sci-

ence Foundation of China with grant (11475159, 11505173,11505174,

11575264, 11605190 and 11105019)† liming@caep.cn

FACILITY COMPONENTS

Overview

Figure 1 shows the layout of the CTFEL facility. High av-

erage power high-brightness electron beam is emitted from

the high-voltage DC gun equipped with a GaAs photocath-

ode. The beam is then energized by a 2×4-cell RF supercon-

ducting accelerator and gain kinetic energy from 6 MeV to

8 MeV. Passing through an achromatic section, the beam

then goes into the undulator magnet field and generates spon-

taneous radiation. The radiation resonates in the THz optical

cavity and reaches saturations.

Figure 1: The layout of the CTFEL facility.

Table 1 gives more information of the electron beam. The

accelerator is designed to operate in both CW and macro-

pulse mode. It is now working in macro-pulse mode in the

first step. The typical pulse duration and repetition rate are

1 ms and 1 Hz, respectively. The duty cycle will upgrade

to >10% in 2018 as the “step two”. And the CW operation

will be reached in the “step three”.

High-voltage DC Electron Source

Figure 2 shows the system of the high-voltage DC elec-

tron source, which consists of a photocathode preparation

chamber, a load-lock system, a drive laser, a high-voltage

DC gun and some beam elements such as three solenoids

and an RF buncher. The electron source can provide 320 keV

high brightness beams both in CW mode and in macro-pulse

mode. The average current has reached 1 mA∼5 mA. The

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02 Photon Sources and Electron AcceleratorsA06 Free Electron Lasers

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Table 1: Electron Beam Parameters

Parameters Design goal Unit

Bunch charge 10∼100 pC

Micro-pulse repetition 54.167 MHz

Macro-pulse repetition 1∼20 Hz

Duty cycle 10−5 ∼1

Kinetic energy 6∼8 MeV

Normalized emittance 0.3 MW

Minimum transverse radius

Pavg =V

0.49Tτrji, (1)

where V is the output voltage of the energy meter, which is

about 1260 mV in Fig. 5 (a). The other parameters are as

follows: The parameter T ≈0.55 is the transmission of the

TPX window. The pre-calibrated rji is about 0.233 mV·µJ−1.

The factor 0.49 means that the absorption of the energy

meter’s metal film is about 49%. The macro-pulse duration

is measured by a GeGa detector, and is 920 µs, as shown

in Fig. 5 (b). All the results indicate that the macro-pulse

average power is about 21 W. The energy of macro-pulse is

about 20 mJ.

Figure 5 (c) shows the electron beam signal for compari-

son, detected by a Fast-Current-Transformer (FCT). Because

of the lack of feed-forward system, the macro-pulse of the

electron beam is longer than the THz macro-pulse.

Figure 5: The THz laser measurement.

The spectrum measured by a Fourier spectrometer (Bruker

VERTEX 80V) at three single spots is shown in Fig. 5 (d).

The frequency is adjusted by both the undulator gap and the

electron energy. Figure 5 (e) is the THz beam transverse

profile captured by a pyroelectric array camera "Pyrocam

IIIHR“ (15 cm downstream the focal point). The beam fulfill

the transverse Gaussian distribution. The minimum beam

size around the focal point is estimated as less than 1 mm in

diameter.

Figure 5 (f) is the "energy versus moving-mirror" curve

in the time domain caught by the Fourier spectrometer.The

spectral FWHM is a little wider than the theoretical simula-

tion because the micro-pulse length is a little shorter, typi-

cally as short as about 400 fs (RMS), caused by the phase

slippage, making the peak power more than 0.3 MW.

An example of high-duty-cycle running is shown in Fig. 5

(g), where the macro-pulse duration is 4 ms and the macro-

repetition rate is 20 Hz, making the duty-cycle about 8%.

Figure 5 (h) shows the THz micro signal captured by a

graphene detector[15, 16], whose rising time is less than

1 ns.

At last, the power density was so strong that it damaged

the terahertz attenuator and the TK energy meter, as shown

in Fig. 6.

Figure 6: The damage caused by the THz laser.

SUMMARY

This paper has briefly introduced the CTFEL facility, the

first THz free electron laser oscillator in China. This facility

mainly consists of the high-brightness high-voltage DC elec-

tron source, the CW RF superconducting accelerator and

the undulator-optical-cavity system. The CTFEL facility

provides monochrome, high-power, linear-polarization, and

frequency-continuously-tunable THz laser. The terahertz fre-

quency is continuously adjustable from 2 THz to 3 THz. The

average power is more than 20 W and the micro-pulse power

is more than 0.3 MW. CTFEL will be working as an user fa-

cility, and will greatly promote the development of the THz

science and its applications on material science, chemistry

science, biomedical science and many other cutting-edge

areas in general.

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02 Photon Sources and Electron AcceleratorsA06 Free Electron Lasers